Rosetta Shape Models of comet 67P/Chuyumov-Gerasimenko - Users Guide ======================================================================== General Terminology -------------------- Because this dataset is a collection of shape models from different providers, using different techniques and different data formats, and given in multiple versions, the terminology used in the documentation can become confusing. We include a description of the general terminology here as a guide to what can be found elsewhere. Not all terms are necessarily included here, but this guide should make it obvious, by analogy, what any missing terms are referencing. Data Providers ---------------- Terms referring to the locations (physical or organizational) that have provided the data products, or contributed to their development. DLR - German Aerospace Center ESA - European Space Agency LAM - Laboratoire d'Astrophysique de Marseille, PSI - Planetary Science Institute RMOC - Rosetta Mission Operations Center Shape modeling Techniques -------------------------- Terms referring to the techniques used to create the shape models. Specific references that provide more information are included in the detailed discussions of the individual models. MSPCD - Multi-Resolution StereoPhotoClinometry by Deformation SPC - Stereophotoclinometry SPG - Stereo-photogrammetric Shape Model Version -------------------- Terms identifying various versions (or generations) of the shape model, some of which may not be available in the archive, yet. These may be subdivided into different resolutions (different numbers of vertices/plates) usually produced by smoothing of a higher resolution into a lower one. These terms tend to refer to the range of dates for which data were used to create the model, and are defined in the SHAPE_DATA_ORGANIZATION.ASC file in the DOCUMENTS directory and in the scientific index in the INDEX directory. SHAP2 - Different generations of the OSIRIS-derived models SHAP4 SHAP4S SHAP5 - PSI/LAM versions of the OSIRIS-derived SPC models SHAP7 MTP009 - ESA/RMOC versions of the NAVCAM model MTP019 Shape Model Formats -------------------- Terminology defining the different format types that are used for the models. Note that all current models are defined by triangular plates produced by connecting cartesian coordinate vertices, but the presentation of the data differs slightly in the various formats. The data label contains the relevant information needed to read the file (except for DSK and STL). Not all models are given in all formats. VRML (.WRL) - ASCII table format adopted by the SBN for general shape models. Described in detail in the PDSSBN_PLATE_SHAPE_DEF.ASC file in the DOCUMENTS directory. Includes a VRML wrapper that allows the model to be displayed with freely available software. .ROS - Format adopted by the Rosetta team. .OBJ - Object format defined by Wavefront Technologies for use in 3D printers, etc. .STL - STereoLithography format native to the stereolithography CAD software created by 3D Systems. Note that this format cannot be easily described in the label, and thus is not included in the DATA directory. Instead, it is included in the EXTRAS directory when provided. .BDS - Ditigal Shape Kernel for use with the SPICE toolkits. Created by converting the PDSSBN version to binary format. ============================================================================== Organization of the dataset --------------------------- Because the dataset combines many different shape models from a number of different sources, the organization of the data is not straightforward. A description of how the data are organized is provided in the SHAPE_DATA_ORGANIZATION.ASC file in the DOCUMENTS directory. ============================================================================== Reference Frame and Rotation State ----------------------------------- The SHAP2, SHAP4S, MTP009 and MTP019 versions of the shape models were all developed with respect to the Cheops reference frame, which is defined and described in the CHEOPS_REF_FRAME_V1.PDF file in the DOCUMENTS directory. The SHAP5 SPC version of the shape models was reconstructed in a frame rotated by 0.28 deg and translated by (2.8,1.3,17.0) m with respect to the Cheops frame. The transformation between the two frames was determined using a tool called "PC_ALIGN" available in the NASA Ames Stereo Pipeline (ASP). The method is based on the article of Pomerleau et al., 'Comparing ICP Variants on Real-World Data Sets', Autonomous Robots 34(3), 133-148, February 2013. The rotation state of the nucleus, for dates prior to early September 2014, is also described in that document. As the nucleus approached the Sun, cometary activity introduced changes in the nucleus' rotation, and a more complete description can be found in the Rosetta SPICE dataset (RO/RL-E/M/A/C-SPICE-6-V1.0). Additional information, where available, can be found in the dedicated shape model information files in the DOCUMENTS directory. SHAP2_MODEL_INFO.ASC SHAP4S_MODEL_INFO.ASC SHAP5_MODEL_INFO.ASC ============================================================================== GUIDE FOR OSIRIS MODELS ----------------------- The SHAP2 archive contains two different models of comet 67P/C-G: the SPC model is reconstructed with the "stereophotoclinometry" technique, and the MSPCD model build with a new method called "multi-resolution stereophotoclinometry by deformation", which are described more fully in the shap2_model_info.asc document. This file provides some guidance to the users of these models. The SHAP4S archive contains an SPG model of comet 67P/C-G: the SPG model is reconstructed with the "stereo-photogrammetric" technique. This is the model that was used to define the Cheops reference frame and the local reference reference frames for the two lobes and the neck. The model and its implications are described more fully in the shap4s_model_info.asc document. The SHAP5 archive only contains the SPC model reconstructed with the "stereophotoclinometry" technique from a much larger set of images than the SHAP2 models. This model has a higher resolution than the SHAP2 models and should be used in priority. The MTP009 archive contains an SPC model of comet 67P/C-G that was developed from NAVCAM images by the flight operations team, primarily for use in navigation of the spacecraft. It is reconstructed with the "stereophotoclinometry" technique using images obtained at the comet between Aug 6 and Nov 2, 2014. Because of the limited observations, the model lacks information regarding the surface in the southern hemisphere. The MTP019 archive contains an SPC model of comet 67P/C-G that was developed from NAVCAM images by the flight operations team, primarily for use in navigation of the spacecraft. It is reconstructed with the "stereophotoclinometry" technique using images obtained at the comet between Aug 6, 2014 and Aug 25, 2015, which allows the Southern hemisphere to be defined. If spatial sampling and vertical resolution information is known for specific models, then this information will be included in the individual model information documents. Notes about the techniques: o The SPC technique is more robust and much better tested than MSPCD. It has been successfully applied to several small bodies and planets. o The SPC technique translates gray-level variations in the images into slopes. The algorithm guarantees that it will almost never create artificial topography. MSPCD on the other hand can sometimes create small "ripples", holes or discontinuities in the topography due to the nonlinear inversion algorithm used to generate it. o Although both techniques use a "smoothness constraint" to regularize the solution (slopes for SPC, heights for MSPCD), the SPC algorithm tends to smooth sharp edges more than MSPCD. This has a real impact on the shape of 67P/C-G since the comet exhibits several features containing such sharp edges: cliffs, large boulders, pits. Recommendations for users: The SHAP5 SPC model should be used in priority. If the user's work relies on an accurate description of the slopes along cliffs and/or pits, the MSPCD model may be used in addition, to compare the results obtained with the SPC model. ============================================================================== COMMENTS REGARDING THE ESA/RMOC NAVCAM MODEL -------------------------------------------- The MTP009 and MTP019 archives contain the model of comet 67P/C-G produced by Flight Dynamics at ESOC using NAVCAM data. They were constructed using the "stereophotoclinometry" technique, described more fully in the shap2_model_info.asc document. These models were created specifically for use in operational applications. They can be used in scientific applications as well, though their strengths and weaknesses in this respect were not specified at the time of the data set ingestion.